Adding tool namespace to better support IDEs

Spring has a special tool namespace that allows developers of custom namespaces to annotate their XML schemas so IDEs like Spring Tool Suite and IntelliJ IDEA can provide code completion and error checking. I have added the tool namespace to the spring-elasticsearch schemas.
The tool namespace has the ability to define what Java types are exported from custom elements and what Java types are accepted in attributes. Here is an example:

```xml
<xsd:element name="node">
    <xsd:annotation>
        <xsd:documentation><![CDATA[ ... ]]></xsd:documentation>
        <xsd:appinfo>
            <tool:annotation>
                <tool:exports type="org.elasticsearch.node.Node"/>
            </tool:annotation>
        </xsd:appinfo>
    </xsd:annotation>
    <!-- ... -->
</xsd:element>
```

Now when the custom namepsace is loaded in an IDE it will recognize the tool elements and provide errors and suggestions. No manual scanning will be required. Let me know if you have any questions.

Closes #43.